I've had an original, a series 4, and now a series 8. The deciding factor of whether or not I upgrade has always been the battery health of my current watch. All three models have had especially amazing battery life when new, usually able to go almost two full days without a charge if powered down or put in low battery mode overnight (the old low battery mode where it just shows the time and nothing else).
My point is that historically my Apple Watches have gotten better than advertised battery life when brand new and pretty far into middle age, and then when they start losing capacity, they seem to dwindle down to where they're barely making it to bed time, which is when I move to a new model.
Question--is this a cellular model and if so, are you using it without the iPhone present a lot of the time? My younger son does not yet have an iPhone but he does have a cellular Apple Watch SE that I manage from my iPhone. On school days when it can't be on wifi here at the house, it is definitely dwindling by the time he gets home, especially on the days he uses it as an iPod a lot.